1. Field of the Invention
The present invention relates to an imaging lens for forming an optical image on a solid state imaging device which is suitable for use for an onboard camera, a monitor camera, a mobile terminal camera and the like and a camera system which utilizes the imaging lens.
2. Description of the Related Art
Onboard cameras are known which pick up images of surroundings of a motor vehicle on which the cameras are installed to show the driver conditions surrounding the vehicle via images on a monitor screen (for example, refer to JP-A-2006-168683). These onboard cameras include, for example, an outside camera which is provided on an outside of the vehicle for assisting the driver to drive the vehicle by picking up images of surroundings which lie within a blind range to the driver and an inside camera which is designed to pick up images of conditions resulting when the vehicle is involved in a traffic accident in the same way as viewed by the driver. In recent years, in association with an increase in performance and a decrease in price of solid state imaging devices, it is becoming a general trend to equip onboard cameras even on ordinary vehicles.
In the related art, imaging lenses like those described in, for example, JP-A-1-265216 and JP-A-2004-145256 are known as imaging lenses for use for monitor cameras. The imaging lens described in JP-A-1-265216 provides an f number of f/1.4, which is bright, and attains an angle of view of 80 degrees or more. The imaging lens described in JP-A-2004-145256 provides an f number of f/2.4, which is relatively bright, and attains a wide angle of view which can provide a semi-angle of view of 60 degrees or more In addition, those described in JP-A-2004-61763 and JP-A-2004-354888 are known as being similar to the imaging lenses described above.
With the imaging lens described in JP-A-1-265216, there is provided a drawback that the field curvature is large. With the imaging lens described in JP-A-2004-145256, there is provided a drawback that the distortion is large, and hence, images picked up by the relevant lens have to be processed to be displayed on a monitor and image quality in a peripheral portion thereof is deteriorated. The imaging lenses described in JP-A-2004-61763 and JP-A-2004-354888 are lenses developed for endoscopes, and their f numbers which are 3.5 or larger present a drawback that the lenses with those f numbers too dark to be used as onboard cameras and monitor cameras. Furthermore, there are circumstances in relation to imaging lenses for onboard cameras that when used as an outside camera, a weathering resistance is required which is good enough to make it difficult for the lens surface to be deteriorated, while when used as an inside camera, a thermal expansion resistance is required which is good enough to bear thermal expansions caused in association with an increase in temperature within the passenger compartment during summer time.